The broad long-term objective is to understand the process of chromosome segregation during eukaryotic cell division. Inaccurate chromosome segregation leads to genetic instability. Down syndrome is the most frequent genetic defect in infants and cancer will kill one American in four. Both are conditions caused by genetic instability and it is possible that lesions in the spindle checkpoint cause meiotic chromosome loss in Down syndrome and chromosome loss and gain in many tumors. The primary focus within the proposal is the study of the molecular mechanism by which chromosomes signal to stop cell cycle progression in response to mitotic spindle damage in budding yeast. The first specific aim is to identify the components of the complex that inhibit Cdc20p-APC activity in response to spindle damage. The second specific aim is to identify the kinetochore sensor necessary for activating the checkpoint signal complex. To achieve these ends, yeast molecular genetics coupled with classical protein biochemistry shall be employed. When used in combination these methods have been shown to be powerful tools for exploring the molecular roles of protein complexes. The hope is that understanding the molecular mechanism of the spindle checkpoint in budding yeast will help us to understand its role in human disease.